Space-time coding (STC) techniques have been extensively studied to provide spatial transmit diversity, which is also referred to as antenna diversity or transmit diversity. A primary goal of spatial transmit diversity is to enhance wireless link reliability, and to meet increasing demands for higher data rates.
In space-time coding techniques, information bits are encoded by a space-time encoder into multiple streams, and each stream is modulated and transmitted via a respective antenna. There are two classes of space-time coding techniques: space-time block coding (STBC) and space-time trellis coding (STTC).
Space-time block coding is a type of space-time coding used in multiple-antenna wireless communication systems. Space-time block coding transmits multiple copies of a data stream across a number of antennas, and exploits the various received versions of the data to improve reliability of the data-transfer. The fact that transmitted data traverses a potentially difficult environment with scattering, reflection and refraction, as being corrupted by thermal noise in the receiver, means that some of the received copies of the data will be better than others.
This redundancy results in a higher chance of being able to use one or more of the received copies of the data to correctly decode the received signal Space-time coding combines all the copies of the received signal in an optimal way to extract as much information from each of them as possible.
An example of a conventional space-time coded communications device 20 with two antennas 22, 24 is shown in FIG. 1. For every input symbol 30 received by the encoder 26, two channel symbols 32, 34 are output. Each channel symbol 32, 34 is then modulated and up-converted into a radio frequency (RF) signal by a respective transmitter 40, 42. The up-converted signals are then transmitted by antennas 22, 24. As shown in the figure, two RF transmitter chains 40, 42 are required for the two-antenna system.
Still referring to the encoder 26, for every two incoming symbols S0 and S1 during an interval of the first symbol interval 30, the communications device 20 transmits symbol S0 from antenna 22 and symbol S1 from antenna 24. During a second symbol interval, symbol S0* is transmitted from antenna 22 and symbol −S1* is transmitted from antenna 24, where the superscript * denotes a complex conjugate. The transmitted symbols can be decoded at the receiver using straightforward linear operations. Due to its simplicity, space-time block coding has been adapted by multiple wireless communication standards, such as cdma2000, UMTS W-CDMA, IEEE 802.11n and IEEE 802.16. However, a problem with space-time block coding is that it does not provide coding gain.
As with space-time block coding discussed above, space-time trellis coding is a type of space-time coding also used in multiple-antenna wireless communications systems. Space-time trellis coding transmits multiple, redundant copies of a trellis (or convolutional) code distributed over time and a number of antennas (i.e., space). These multiple, diverse copies of the data are used by the receiver to reconstruct the actual transmitted data. For a space-time code to be used, there must necessarily be multiple transmit antennas, but only a single receive antennas is required. Nonetheless, multiple receive antennas are often used since the performance of the communications system is improved.
In contrast to space-time block coding, space-time trellis coding provides both coding gain and diversity gain and has a better bit-error rate performance. However, since space-time trellis coding is based on trellis codes, it is more complex to encode and decode than space-time block coding. Space-time trellis coding relies on a Viterbi decoder at the receiver, where space-time block coding needs only linear processing.
The output of the encoder in space-time trellis coding is thus determined by a trellis structure. The status of the trellis is fully characterized by its state. A state transition is incurred by every incoming information symbol. Along with the state transition, multiple channel symbols are output. Each channel symbol is then modulated and transmitted by an antenna. By properly designing the trellis structure, both diversity and coding gain can be achieved. Space-time codes for wireless communications systems is disclosed in greater detail in an article titled “Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction,” by Tarokh et al., which is incorporated herein by reference in its entirety.
As shown in FIG. 1, each transmitter 40, 42 is directly coupled to a respective antenna 22, 24. The communications device illustrated in the Tarokh et al. article also has a respective antenna directly coupled to a transmitter. For these space-time coding systems using spatial transmit diversity, there is a cost and power consumption penalty for each transmitter. For battery operated communications systems utilizing spatial transmit diversity, extending the operating time of the battery, as well as reducing costs, are important concerns that need to be addressed.